Canadian tar sands, commonly called oil sands, are a combination of clay, sand, water, and bitumen, heavy black viscous oil. Oil sand, as mined commercially, typically contains an average of 10-12% bitumen, 83-85% mineral matter and 4-6% water. A film of water coats most of the mineral matter, and this property permits extraction by a hot-water process.
The hot water process is a common commercial process used for extracting bitumen from mined oil sands. The oil sand is put into massive rotating drums and slurried with hot water (50-80° C.) and some steam. Droplets of bitumen separate from the grain of sand and attach themselves to tiny air bubbles. Conditioned slurry is passed through a screen to remove rocks and large pebbles and pumped into large, conical separation vessels where a froth of bitumen is skimmed from the top containing about 60% bitumen, 30% water and 10% solids. The coarse sand settles and is pumped to disposal sites. Some of the smaller bitumen and mineral particles remain in an intermediate water layer called middlings and are pumped to separation vessels. Approximately 90% of the bitumen in the mined oil sands is typically recovered.
The recovered bitumen generally needs to be upgraded to convert the heavy viscous bitumen to a form which can be transported in existing pipeline systems and to ensure an upgraded crude quality which will permit existing refineries to meet anticipated market product demand. The Flexicoking™ followed by hydro-treating of the coker liquids is typically the preferred upgrading process in Canadian tar sand operation.
The production of one barrel of synthetic crude (upgraded bitumen) through the hot water process typically requires about 4.5 barrels of water. Almost all of the water withdrawn for oil sands operations ends up in tailings ponds. Both primary and final extraction plant tailings are pumped to the retention pond for storage.
When these effluent streams containing bitumen, naphtha, water, and solids are discharged to the pond, a portion of the residual bitumen and diluents naphtha floats to the surface of the pond. The dense sand fraction present in the primary stream settles rapidly but the lighter water fines suspension settles very slowly, forming a zone of sludge. After a period of settling a shallow layer of relatively clear water develops near the surface of the pond. Water from this layer is recycled to the extraction process. But the majority of water remains in this sludge, a water-bitumen-fine solids emulsion that is very difficult to break.
The processing of bitumen into synthetic crude through the hot water process requires energy, and this energy is usually generated by burning natural gas which releases greenhouse gas. For example, the production of 1 barrel of synthetic oil may necessitate approximately 1.0 to 1.25 gigajoules of energy and can lead to the release of more than 80 kg of greenhouse gases into the atmosphere.
Thus, the hot water process can lead to problems due to large water requirements, disposal of large tailing ponds, greenhouse gas production and large requirements of energy are major problems facing the oil sand industry.
As such, improvements in the extraction of oil from oil sand or tar sand are desirable.